Steel sheets used after pressing in automobiles and industrial machines are required to possess both high strength and high ductility, which requirement has been becoming more and more strong in recent years.
Heretofore, as a steel sheet having both high strength and high ductility there has been known a composite ferrite-martensite steel sheet [dual phase (DP) steel sheet] comprising ferrite as a base and a low temperature transformation structure contained therein which structure is constituted mainly by marternsite (see, for example, JP-A No. 122820/1980). This steel sheet is not only superior in ductility but also characteristic in that yield elongation does not appear due to a large quantity of free dislocation introduced into a martensite producing region, and yield stress becomes lower, and that therefore a shape freezing characteristic in working is satisfactory. By making control to the aforesaid structure there is obtained a steel sheet high in tensile strength (TS) and superior in elongation (El) characteristic, but inferior in stretch flange formability [hole expanding property (local ductility)].
On the other hand, as a steel sheet superior in stretch flange formability there is known a two-phase steel sheet of ferrite and bainite (see, for example, JP-A No. 145965/1982). This steel sheet, in comparison with the above DP steel sheet, is superior not only in stretch-flange formability but also in resistance-weldability (especially there is no softening of a heat affected zone) and in fatigue characteristic. However, there is a problem that the steel sheet in question is inferior in elongation characteristic.
Further, there is known a retained austenite steel sheet wherein retained austenite (γR) is produced within the structure and undergoes induced transformation (strain induced transformation: TRIP) during deformation in working to improve ductility. For example, JP-A No. 43425/1985 discloses a steel sheet which is high in strength and extremely superior in ductility and which is produced by controlling the structure of a composite phase steel sheet into a structure having 10% or more of ferrite and 10% or more of γR in terms of volume fraction, with the balance being bainite or martensite or a mixture thereof. It is described in the above unexamined publication that with such a structure, not only the strain induced transformation effect of γR but also high ductility is exhibited by soft ferrite, resulting in ductility being ensured by ferrite and γR and strength ensured by bainite and martensite. However, also in the case of this steel sheet, like the foregoing DP steel, there has been a problem of stretch flange formability being unsatisfactory.
In view of the above-mentioned problems, studies have been made for providing a steel sheet superior in such formability as stretch flange formability (hole expanding property) while ensuring good strength-ductility balance based on γR. JP-A No. 104947/1997 discloses a steel sheet having a three-phase microstructure of ferrite, bainite and γR and with a ferrite occupancy rate/ferrite grain size ratio and γR occupancy rate being controlled to predetermined ranges. This is based on the following knowledge: “An increase of γR brings about improvement of strength-ductility balance and of total elongation and the effect thereof is enhanced by microstructurization; further, as γR becomes finer, formability such as stretch flange formability is also improved.” However, the improvement in stretch flange formability is low and it is keenly desired to provide a high strength steel sheet further superior in stretch flange formability.
Further, for the application of a high strength steel sheet to automobile components, especially such structural members as automobile body members and frames or suspension members such as suspensions and wheels, it is required for the steel sheet to be superior not only in the foregoing elongation and stretch flange formabilitybut also in fatigue characteristic [fatigue endurance ratio (fatigue strength/yield strength)]. Generally, low alloy TRIP steels involve the problem that their fatigue characteristics are deteriorated by martensite of a second phase structure (martensite resulting from transformation of retained austenite).
Further, in applying a high strength steel sheet to suspension members of an automobile as referred to above, it is required for the steel sheet to be superior in bake hardening property (BH property). As to this BH property, it is presumed that, by baking finish after working, C (solid solution C) dissolved supersaturatedly in ferrite is fixed to dislocation in the ferrite which has been introduced during working, with consequent increase in yield strength of the steel sheet, thus leading to improvement of BH property.
However, since there is a limit to the amount of the solid solution C capable of being present supersaturatedly in ferrite, it is difficult to attain a predetermined or higher BH property. For example, there is a problem such that a large deformation results in marked deterioration of BH property, not affording a sufficient strength. For example, also in JP-A No. 297350/2000 there is disclosed a high tensile strength hot-rolled steel sheet, but the present inventors have found out that BH (10%) is about zero although BH (2%) is high.